Just doing a quick pop-in to check if GC integration is a long-term goal (and whether I should direct someone interested in this issue towards this WG). Per the linked RFC: "Regarding GC: We plan to allow future allocators to integrate themselves with a standardized reflective GC interface, but leave specification of such integration for a later RFC. (The design describes a way to add such a feature in the future while ensuring that clients do not accidentally opt-in and risk unsound behavior.)"
It’s not at all clear to me what such integration means. Please direct that person to open an issue on https://github.com/rust-lang/wg-allocators/issues to discuss it